1. Technical Field
The present invention relates to an optical disk drive, and more particularly to a technique for compensating for a phase shift in a wobble signal ascribable to crosstalk originating from adjacent tracks.
2. Related Art
In an optical disk such as a DVD+R/+RW, an HD DVD-R/-RW, or the like, a wobble signal is recorded by means of wobbling a groove. The wobble signal is modulated and recorded by means of a two-phase modulation mode employing a phase value of 0° and a phase value of 180°. Since the wobble signal is used for controlling the rotation speed of an optical disk or modulation of an address, the optical disk drive must extract a wobble signal from a signal reproduced by an optical pickup without fail.
In order to increase recording density, track pitches of an optical disk are formed so as to become sequentially narrower. A portion of a beam spot is radiated on an adjacent track as well as on a target track. As a result, crosstalk originates from the adjacent track. A detected wobble signal tends to become a signal where a wobble signal of an adjacent track is superimposed on a wobble signal of the target track.
When a wobble signal is subjected to binary phase modulation, the influence of crosstalk is not uniform and varies with a relationship between the phase of the wobble signal of the target track and the phase of the wobble signal of the adjacent track. Specifically, when the target track and an adjacent track are in phase with each other, amplitudes of detected wobble signals reinforce each other, to thus become greater. In contrast, when the wobble signal of the target track and the wobble signal of an adjacent track are out of phase with each other, the amplitudes of the detected wobble signals weaken each other, to thus become smaller. In addition to the changes in the amplitude of the wobble signal, a shift also arises in the phase of the wobble signal under influence of crosstalk.
FIG. 4 shows a waveform of a basic wobble signal (a wobble signal which is not affected by the crosstalk signal), a waveform of a crosstalk signal, a waveform of a 0-degree phase wobble signal affected by the crosstalk signal, and a waveform of a 180-degree phase wobble signal affected by the crosstalk signal. In the drawing, the horizontal axis represents time, and the vertical axis represents amplitude. The phase of the 0-degree phase wobble signal appearing in a segment A in the drawing is advanced by the crosstalk signal, and the phase of the 180-degree phase wobble signal appearing in a segment B is delayed by the crosstalk signal. Consequently, the wobble signal is originally demodulated at a phase value of 0 degree or 180 degrees. However, the phase of the wobble signal is shifted from a value of 0 degree or a value of 180 degrees, and hence difficulty is encountered in demodulating the wobble signal.
For instance, in Japanese Patent Laid-Open Publication No. 2005-4889, fluctuations in the amplitude of a wobble signal attributable to crosstalk are corrected by use of two amplifiers; namely, a fixed gain amplifier and variable gain amplifier.
When the phase of a wobble signal is detected, it is better to generate a reference clock signal from the wobble signal by means of a PLL circuit and compare the phase of the wobble signal and the phase of the reference clock signal. For instance, an exclusive-OR (EX-OR) result of the wobble signal and the reference clock signal is computed, and a determination is made as to whether or not the result assumes a value of 0 or 1, thereby determining whether or not the phase of the wobble signal is 0 degree or 180 degrees. In short, when the exclusive-OR result assumes a value of 0, the phase of the wobble signal is determined to be a value of 0 degree. When the exclusive-OR result assumes a value of 1, the phase of the wobble signal is determined to be 180 degrees.
However, as shown in FIG. 4, when crosstalk originating from an adjacent track is superimposed on the wobble signal, the phase of the 0-degree phase wobble signal and the phase of the 180-degree phase wobble signal are shifted, so that the value of exclusive-OR result changes in a complicated manner. Specifically, on the assumption that the period of a reference clock signal is taken as a reference, the exclusive-OR result assumes a value of 0 or 1 in a one-half period of the reference clock signal unless there is a phase shift. However, when there is phase shift, the exclusive-OR result assumes both a value of 0 and a value of 1 in one-half period of the reference clock signal. If there is no phase shift, the ratio of a value of 0 or 1 achieved in one-half the period of the reference clock is 100%. However, when there is a phase shift, the ratio decreases. A determination as to whether the phase of the wobble signal is 0 degree or 180 degrees can be usually made by means of taking a phase value of 90 degrees (a ratio of 50%) as a criterion for determination. When the phase value is less than 90 degrees, the phase can be determined to assume a value of 0 degree. When the phase value is in excess of 90 degrees, the phase can be determined to assume a value of 180 degrees. However, occurrence of phase shift leads to a decrease in the accuracy of determination.